The past year was a rough one for the climate, so why am I so hopeful about climate disruption?

Here in the United States, last year was officially the hottest on record, with wildfires, droughts and a superstorm that devastated parts of the Northeast.

And the United States was not alone in its climate misery. Near-permanent flood conditions in Bangladesh displaced hundreds of thousands of people. Russia, China and Indonesia also experienced massive floods in 2012. Arctic sea ice reached another record low.

So it was both fitting and welcome that President Barack Obama, in his inaugural address, named addressing climate disruption as both a top priority for his administration and a moral imperative for humankind. If we care at all about the world we bequeath to future generations, we cannot ignore this simple fact: Either we leave at least two-thirds of proven fossil-fuel reserves in the ground, or we alter our planet's climate beyond all recognition.

Al Gore called this an "inconvenient truth," but "unthinkable outcome" might be a better way of thinking about it. We simply cannot -- must not -- allow the worst to come to pass.

The Sierra Club is committed to doing absolutely everything it can to push Obama to follow through on his Inauguration Day eloquence and forcefully lead our nation (which will in turn lead the world) in responding to the climate challenge.

On February 17, in Washington, we and our allies in the climate movement will hold the largest climate rally in U.S. history. That day will mark a new crest in the rising tide of public demand for action, beginning with a definitive rejection of the Keystone XL pipeline that would enable the exploitation and export of Canada's tar sands oil.

No one in Washington, from Congress to Pennsylvania Avenue, will fail to understand that Americans take seriously their moral obligation to address the climate crisis. But, important as it is to acknowledge the reality of climate disruption, it forms only part of the picture. The other, more welcome, face of this crisis is its enormous opportunity. We already know how to meet this challenge. In fact, we've already begun to succeed -- and to an extent that too few people know about.

For instance, did you know that the United States has led the world in reducing carbon emissions for the past six years? Or that our carbon dioxide emissions have declined to 1992 levels and are still trending lower? The biggest reason for this extraordinary development is our move away from the dirtiest energy source on the planet: coal. Although it once dominated electricity generation in the United States, coal now supplies 40% or less of our power and is projected to be at 30% by the end of this decade.

The Sierra Club's Beyond Coal campaign, which began a little more than 10 years ago in response to the Bush administration's plan to build hundreds of coal-fired power plants, has successfully opposed the vast majority of proposed new plants and helped to retire 130 of the dirtiest, most-polluting existing coal plants in the United States.

During the past three years, we have secured the retirement of a staggering 50,000 megawatts of coal. That's equivalent to the annual carbon emissions from nearly 60 million cars. The benefits aren't limited to climate, either. Taking 50,000 megawatts offline saves lives and improves our quality of life. Coal pollution causes more than 13,000 premature deaths each year in the United States, and according to a 2011 study by the Harvard School of Public Health, the annual health costs of coal exceed $220 billion.

But important as 50,000 fewer megawatts of coal may be, equally significant are the more than 55,000 megawatts of clean wind and solar energy that are online in the United States. Both the wind and solar industries have been growing dramatically. Even more encouraging, though, is the huge, still largely untapped potential for renewable technologies. Last year, a study by the Energy Department's National Renewable Energy Laboratory concluded that renewable energy sources "could adequately supply 80% of total U.S. electricity generation in 2050."

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Here is where the fears provoked by a climate crisis must be matched to the hope promised by a clean-energy future. Our urgent need to stop burning fossil fuels necessarily means we will move faster to develop and adopt renewable technologies that can deliver the energy we need without the toxic consequences that have been inseparable from fossil fuels since the Industrial Revolution. As we leave fossil fuels behind, we will find ourselves "forced" to live in a world with cleaner air and water, better health and unspoiled wilderness.

The president noted that the path toward clean energy "may be long and sometimes difficult." But in the end, it will be a story not of sacrifice but of salvation. We've already taken our first steps toward a better future. All we need to do now is stay on the path.

This means holding polluters accountable, rejecting proposals to import dirty fuels such as tar sands crude, stopping the rush of fossil fuel exports, doubling down on the production of clean energy and protecting our people, land, water and wildlife from the consequences of fossil fuel development and climate disruption.